1. Field of the Invention
The present invnetion relates to a successive comparison type A/D converter, and more specifically to an A/D converter including a reference voltage generator circuit, a comparator, and a control circuit for controlling the former circuits.
2. Description of the Prior Art
In prior practice, an A/D converter is included in a microcomputer, and is connected to a central processing unit (CPU), a DRAMC, and a DMAC through an address bus, a data bus, and a bus access controller, as illustrated in FIG. 5. The A/D converter is further connected to a RAM, timers, and an external port P6, etc., through the address bus and the data bus, for converting analog signals from external apparatuses to digital signals and outputting the converted digital signals to the CPU and the like.
The A/D converter comprises a reference voltage generator circuit for generating a reference input voltage from a reference power supply V.sub.REF, a charge balancing comparator including an amplifier circuit and driven by switching means and an A/D conversion power supply AV.sub.CC, and a control circuit for controlling the above circuits. In the A/D converter, the reference input voltage from the reference voltage generator circuit is altered by the control circuit and inputted into the charge balancing comparator alternately with an analog input voltage for successive comparison with the latter, and there is estimated a changing point of an output voltage from the comparator with respect to the analog input voltage for converting the analog signal to a digital signal (A/D conversion).
With the prior A/D converter constructed as described above, a difference is produced between fine changes in the voltage of the reference power supply V.sub.REF and the voltage of the A/D conversion power supply AV.sub.CC in the neighborhood of the time the input into the charge balancing comparator is switched from the analog input voltage to the reference input voltage. Accordingly, there is produced a difference, as an error as a result of the A/D conversion, between a shift of the A/D conversion from a correct one due to the fine change in the reference power supply of the reference voltage generator circuit and that due to the fine change in the A/D conversion power supply for an amplifier circuit of the charge balancing comparator. Hereby, accuracy of the A/D conversion is lowered.